Dynamic Nucleation in Sub-Critically Undercooled Melts During Electromagnetic Levitation

By classical nucleation theory, subcritically undercooled melts in electromagnetic levitation are expected to be very stable for extended periods of time. However, there have been cases in which the molten sample demonstrated nucleation in conditions that are inconsistent with classical nucleation theory. During experiments on pure zirconium during the MSL-1 electromagnetic levitation campaign onboard the space shuttle in 1997, the sample was observed to solidify during an isothermal hold at subcritical undercoolings. Additionally, a Zr 64 Ni 36 sample solidified concurrently with an excitation pulse during experiments in the IML-2 space shuttle campaign in 1994. Both anomalous nucleation events have been attributed to dynamic nucleation. More recent experiments have both replicated the results of both sets of experiments. The conditions of the MSL-1 solidification events were replicated in the ISS-EML using a pure zirconium sample held between 45°C and 290°C below the melting temperature. The sample successfully solidified in 18 of these experiments in less than 600 seconds. The IML-2 result, was replicated in the ISS-EML in which a Zr 64 Ni 36 sample was used to demonstrate pulse-triggered nucleation events between 59.5°C and 64.5°C below the melting temperature. The results of both sets of experiments are consistent with the theory that the nucleation events are affected by the flow conditions within the drop.